Bunch of improvements on Autofill samples:

- Created CreditCardAntiPatternActivity that uses text field for credit card
  expiration date.
- Renamed CreditCardAcitivity to CreditCardSpinnersActivity.
- Created a new CreditCardAntiActivity that uses text field for individual
  credit card expiration date fields (day, month, year).
- Added a helper method to dump an AssistStructure.
- Changed CustomVirtualView to accept fields of type AUTOFILL_TYPE_DATE.
- Changed MultiplePartitionsActivity to use either date or text for credit card
  expiration dates.
- Removed invisiblity property from CustomVirtualView lines.

Bug: 65384186
Bug: 65673410

Test: manual verification

Change-Id: I02e80a87ce14792dbd7c50b1c9e9665be38babb1